1. Field of the Invention
This invention relates generally to the preparation of microfluidic chips and particularly those chips fabricated from thermoplastic materials.
Thermoplastic polymers generally comprise the set of linear polymers that are not cross-linked and are therefore free to flow as a liquid above a certain temperature. This class of polymers includes polyethylenes, polypropylenes, vinylidene chloride copolymers, polystyrenes, polycarbonates, polymethylmethacrylates (“PMMA”), polyvinylchloride, polyoxymethylenes, polysulfones, polyetheretherketones, polyamides, polyphenylenes, cyclic olefin copolymers, and many others. Of these, Cyclic Olefin Copolymers (hereinafter “COCs”) comprise a subset of thermoplastic polymer materials characterized by a usefully high glass-transition temperature (hereinafter “Tg”), high transparency, low birefringence, high rigidity, strength and hardness, low water absorbency, and good resistance to acids and alkalis making these materials particularly well suited for use in fabricating diagnostic equipment and devices that rely on visible and ultraviolet (hereinafter “UV”) light for interrogation techniques.
Several types of cyclic olefin copolymers are available commercially based on different types of cyclic monomers and polymerization methods. A review of their structure and physical properties is presented in a technical report by Shin, et al., published in IUPAC journal of Pure and Applied Chemistry, v. 77 (5), pp. 801-814, 2005 and is herein incorporated by reference. In particular, COCs such as ZEONOR®, (a product of the Nippon Zeon Co., Ltd. Corporation Japan, Tokyo, Japan, available from its American subsidiary Zeon Chemicals L.P., Louisville, Ky.) show utility as substrates for microfluidic devices. The thermal properties of this class of plastics may be tailored to fit desired application specifications and are compatible with standard molding and stamping techniques. Furthermore, the optical properties of these materials (i.e., negligible bulk fluorescence and amorphous structure with low intrinsic light scattering) allow the detection of very weak sample fluorescence signals in microfluidic devices fabricated from these plastics. Finally, the material bulk properties of these materials result in robust, resilient microfluidic devices capable of surviving extended field-use, and although their surface properties are not ideal, they nevertheless accommodate a variety of water-based analytical methods.
This application addresses the challenges that arise in assembling COC parts into multilayer structures where it is necessary to produce microfluidic devices comprised of two or more COC pieces (hereinafter ‘multilayer structures’). In order to do so, pieces must be joined, preferably in a manner that forms a bond line whose mechanical strength is comparable to that of the bulk material. Moreover, during the joining process, any distortions of features formed in the surfaces of the device to be joined must be minimized in order to avoid compromising the device function.
COC multilayer structures are currently produced using thermal bonding procedures, wherein the parts to be joined are aligned and superimposed, and then heated under pressure until a bond forms. Unfortunately, under conditions necessary for forming an acceptably strong bond, COC polymers undergo significant plastic deformation resulting in distortion or full collapse of micro-features previously formed in one or both bonded surfaces.
Bond formation and plastic deformation are both macroscopic manifestations of the same underlying molecular chain motion; a process which promotes the former will inevitably cause the latter to some degree. In particular, the characteristic time constants for ZEONOR® 1060 plastic deformation at bond-promoting temperatures (i.e., between about 88° C. and 100° C.) are significantly shorter than minimal bonding times dictated by thermal conductivity and process constraints, suggesting that significant feature distortion is inevitable during thermal bonding. Similar situations are likely to hold for analogous COC materials.
2. Related Art
Published U.S. Patent Application Serial Number 2004/0084402 discloses a polymer-based microfluidic device and a method for producing same. This device is suitable for chromatographic and electrophoretic separations in which the detection of the components in the fluid is by means of ultraviolet (UV) spectroscopy. The device further comprises a planar substrate (body) and cover that are bonded together wherein at least first and second intersecting channels disposed at the interface of the bonded surfaces. The application teaches a method, wherein a first injection-molded plaque of COC (with or without microfluidic structures) is placed on top of a second plaque having microfluidic structures. The two pieces are subjected to an axial load of 500 psi to 1000 psi at 75° C. for several minutes. The two pieces are then allowed to cool to about at least 60° C. before the pressure is released. This results in a fluid-tight bond between the two plaques. The application also teaches to pre-treat at least one of the plaque surfaces with polyvinyl alcohol layer before pressure is applied.
Published U.S. Patent Application Serial Number 2005/0130292 is directed to an apparatus and method for placing two microfluidic components in fluid communication at an arbitrary position and time, both of which are externally defined. The device comprises a sandwich of at least three layers: a top side, a bottom side, and an intermediate material layer separating the top and bottom sides and permanently joined to each therebetween. As described by the application, joining is achieved by lamination, hot bonding, UV bonding, and plasma treatment of the surfaces, solvent bonding, pressure adhesive, and heat adhesives. The bonding procedure may exploit polymer foils treated with thermoset films on both sides. The inventive apparatus uses electromagnetic radiation to perforate a material layer having selected adsorptive properties. The perforation of the material layer allows the fluid communication between microfluidic components.
Published U.S. Patent Application Serial Number 2005/0151371 is directed to an Injection molding process used to form microfluidic devices with integrated functional components. One or more functional components are placed in a mold cavity which is then closed. Molten thermoplastic resin is injected into the mold and then cooled, thereby forming a solid substrate including the functional component(s). The solid substrate including the functional component(s) is then bonded to a second substrate which may include microchannels or other features As described by the application, joining is achieved by hot die bonding, thermal diffusion bonding, solvent bonding, infrared welding, ultraviolet irradiation, ultrasonic welding, or other joining technologies known in the art, or combinations thereof.
Other examples exist but none exhibit the essential characteristics of the instant invention.